I had this problem after getting the latest version of SimPE. What I discovered is that the new version takes longer to load and that if you click on Sim Browser to soon all the recently updated(played) Sims show up as Unknown. Clearing SimPE's cache and restarting SimPE usually fixes the problem. In the future when loading SimPE keep an eye on the lower left corner to make sure SimPE has finished loading the Neighborhood before proceeding.